Update kerberos.md to include a link to a Kerberos Explainer

Merged Lee Matos requested to merge patch-1 into master

Was on a call with a customer today and we were talking about Kerberos resources. They linked this and I think this is the best spot to share it. Kerberos is an EE feature so this is the right repo. ;) (For posterity, I always mess that up! 💩)